Pressure release valves for aerosol cans

ABSTRACT

A pressure release valve is located in the domed bottom of an aerosol can. A centrally located area in the domed bottom having a lesser resistance to eversion than the peripheral area in the domed bottom includes several score lines which meet at a common point in a spoke-like configuration. The centrally located area of the bottom will evert at a pressure of 170 to 200 p.s.i. so as to rupture the bottom at the source lines before any substantial eversion of the peripheral area occurs.

O 1 United States Patent 1191 1111 3,826,412 Kneusel July 30, 1974 1 PRESSURE RELEASE VALVES FOR 3,356,257 12/1967 Eimer 220/89 B OSO CANS 3,388,601 6/1968 lngham 3,515,308 6/1970 Hayes et al.. [75] in entor: gaymond H- P- K e o n, 3,622,051 11/1971 Benson 222/397 FOREIGN PATENTS OR APPL1CAT1ONS 1 Assigneel r & Seal p y -t 748,953 5/1956 Great Britain 220/89 A Ph1ladelph1a, Pa. [22 Fil d; Ju|y 26, 72 Prir nqry Examiner-Robert B. Reeves Asszstant Examiner-Francs J. Bartuska [21] Appl- N05 2751370 Attorney, Agent, or Firm-Woodcock, Washburn,

Related US. Application Data Kurtz Mackiewlcz [63] (ljgyltinuation-in-part of Scr. No. 199,929, Nov. 18, ABSTRACT I A pressure release valve is located in the domed bot- [52] US. Cl. 222/397 m f n aerosol A ntrally l at d ar a in th [51] int. Cl 865d 83/14 domed tt m having a l ss r resistance t rsi n [58] Field of Search 222/396, 397, 541; than th peripheral ar a in the d m d ttom includes 220/89 A, 89 B, 44 R, 44 D; 137/68-7] Several score lines which meet at a common point in a spoke-like configuration. The centrally located area of [56] References Cited the bottom will evert at a pressure of 170 to 200 p.s.i. UNITED STATES PATENTS so as to rupture the bottom at the source lines before 7 95] 6| 4 9/960 Grccncv 222/397'UX any substantial eversion of the peripheral area occurs. 3,292,826 12/1966 Abplanalp 222/397 '20 ;1 ai 1 ,1) r in g j mamzumm 3.826.412

MET 3 0F 4 PRESSURE RELEASE VALVES FOR AEROSOL CANS RELATED APPLICATION This is a continuation-in-part of copending application Ser. No. 199,929 filed Nov. 18, 1971.

BACKGROUND OF THE INVENTION 1. Field of the Invention Cans of the aerosol type having an internal propellant, such as Freon, are potentially dangerous. When such a container is subjected to excessive temperatures, dangerously high pressure levels are generated within the container. Under these conditions, the can bottom will evert and explode and portions of the can may be propelled at dangerously high velocities. Such conditions may occur when the can is incinerated and even during storage at high temperatures. In order to provide for the safe release of a pressure build-up within the can, a safety valve must be provided.

2. The Prior Art The prior art has suggested the use of scoring to provide pressure release valves in various areas of an aerosol can. US. Pat. No. 3,074,602 Shillady et al. discloses a pressure release valve which is formed by scoring the valve cup of an aerosol can. The scoring supposedly occurs at a point of maximum deformation when the can is subjected to abnormally high pressure levels within. This distortion of the valve cup in the area of weakness is intended to rupture the cup so as to safely release the pressure build-up. A circular score line and a score point are disclosed in two different embodiments of the Shillady et al. patent. I

US. Pat. No. 3,292,826 Abplanalp discloses a pressure release valve formed by at least partially circumscribing a line of weakness, e.g., a score line, over a relatively large section of an aerosol can wall. As suggested in the Abplanalp specification, the line of weakness may be located at the various portions of the container but should not be located on the bottom wall since the discharge of pressure against-the surface on which a conventional can rests would, even with the Abplanalp valve, tend to displace the can from that surface and this would be objectionable.

The particular pressure release valves described in the above-discussed patents have a number of disadvantages. The failure of the valves to release pressure build-ups within a narrow pressure range of 170 p.s.i. to 200 p.s.i. is juat one of these disadvantages. Although the Shillady et al patent does indicate an effective range of 175 to 200 p.s.i. for the valve, such a valve is not consistently efiective in this range. The reason for this is in large part due to the small size of the Shillady scored area which requires a very small residual thickness and this small thickness results in an unreliable release of pressure.

The Abplanalp circular score line which is located in the cylindrical wall of the can body also requires a very small residual thickness, e.g., 0.001 inches assuming a circular score line diameter of one inch. This small residual thickness would be required since there is little or no deformation of the cylindrical wall outside the score line. However, such a small residual thickness is difficult to achieve in commercial production due to inclusions and variations in the metal thickness. The result is the unreliable release of pressure from the can.

Since pressure approaching 200 p.s.i. will result in the eversion of most aerosol can bottoms with the resulting unraveling of the double seam along the periphery of the bottom, it is imperative that the pressure release occur before this dangerously high pressure is reached.

It is of course just as important to avoid a premature release of pressure; i.e., a release below the 170 p.s.i. level. This is extremely important since a pressure of perhaps p.s.i. may be reached during normal operation without any real danger of explosion at that pressure level. Therefore, any release of a pressure build-up of 160 p.s.i. may result in the unnecessary destruction of an aerosol can and its contents.

Another disadvantage of the prior art pressure release valves relates to the aesthetic properties of the aerosol can. Readily visible score lines which appear on the exterior of the can are not pleasing to the eye. This is particularly true where the score line is located on the exterior side of the can side wall and extends over a large area as shown in Abplanalp patent.

The manufacturing technique which is utilized in making the pressure release valves in the Shillady et a1 patent represnts a further disadvantage. The Shillady et al patent discloses the use of a tungsten inert gas shielded electric arc to score the valve cup of the aerosol can. As the Shillady et al patent cautions, this technique may result in excessive scoring so as to create a hole in the valve cup. Furthermore, this method of scoring is relatively costly and therefore not particularly suitable for the can industry where low cost high speed production is required.

SUMMARY OF THE INVENTION safety valve in an aerosol can which does not adversely affect the aesthetic properties of the can.

It is also an object of this invention to provide a safety valve for an aerosol can which is inexpensive to manufacture.

It is still a further object of this invention to provide a safety valve in an aerosol can bottom which does not result in the propelling of the can upon release even when the can rests upon its bottom.

In accordance with these and other objects of this invention, a safety valve is located in the domed bottom wall of an'aerosol can. The valve comprises a central everting area having a resistance to eversion less than that of the integral peripheral concave area surround:

lines which are preferably located on the exterior of the bottom may also be of the compound type.

In a first embodiment, the everting area of the can bottom isflat to provide a lesser resistance to eversion. In a second embodiment, the everting area has a different radius of curvature than the surrounding area and is circumscribed by a line of weakness to provide a lesser resistance to eversion.

In a third embodiment of the invention, the everting area includes a plurality of arcuate score lines located between said linear score lines but spaced therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of an aerosol can of the type embodying the invention;

FIG. 2 is a sectional view of a domed bottom which may be double seamed to the cylindrical side wall portion of the can of FIG. 1 in accordance with one embodiment of this invention;

FIG. 3 is a plan view of the exterior side of the can bottom shown in FIG. 2 showing the safety valve of this embodiment; I

FIG. 4 is a sectionalv view of the can bottom of FIGS. 2 and 3 after release by the valve;

FIG. 5 is a partial plan view of the exterior side of the can bottom of FIGS. 2 and 3 after release by the valve;

FIG. 6 is a sectional view of a score line taken along line 6-6 in FIG. 3;

FIG. 7. is a sectional view of another type of score line;

FIG. 8 is a plan view of the exterior side of a can bottom showing a modified score line configuration;

FIGS. 9 (a-e) depict a method of making the safety valve in the embodiment of FIGS. 1-6;

FIG. 10 is a plan view of the rough scoring taken along line 10-10 of FIG. 912;

FIG. 11 is a plan view of the fine scoring punch taken along line 11-11 in FIG. 9d;

FIG. 12 is a plan view of the fine scoring anviltaken along line 12-12 of FIG. 9d;

FIG. 13 is a sectional view of a domed bottom which may be double seamed to the cylindrical side wall portion of the can of FIG. 1 in accordance with another embodiment of this invention;

FIG. 14 is a plan view of the exterior side of the can bottom shown in FIG. 2 showing the safety valve of this invention; and

FIGS. 15 (a-e) depict a method of making the safety valve in the embodiment of FIGS. 13 and 14;

FIG. 16 is a plan view of another can bottom with another safety valve embodiment of the invention;

FIG. 17 is a partial sectional view of the can bottom shown in FIG. 16 taken along line 17-17',

FIG. 18 is a plan view of a scoring punch which may be utilized to form the safety valve shown in FIGS. 16 and 17;

FIG. 19 is a sectional view of the scoring punch taken along line 19-19 and shown as engaging the can bottom of FIGS. 16 and 17; and

FIG. 20 is a sectional view of the scoring punch of FIG. 18 taken along line 20-20 and shown as engaging the can bottom of FIGS. 16 and 17.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT An aerosol can as shown in FIG. 1 comprises a side wall portion 10 and a top portion 12 receiving a valve assembly including a valve cup 14, a valve stem 16, and a nozzle 18 adapted to be pushed by a finger at a surface 20. A domed bottom wall portion 22 is double seamed to the bottom edge of the side wall portion 10.

In accordance with one embodiment of this invention as shown in FIGS. 2 and 3, the aerosol can of FIG. 1 is provided with a safety valve comprising a scored flat area 24 which is centrally located in the domed bottom 22 and surrounded by outwardly concave peripheral area 26. The several score lines 28 (a-h.) in the flat area 24 meet at a common point 30 to form a spoke-like design. The score line pairs 28a and 28e, 28b and 28f, 28c and 28g, and 28d and 28h are located on diametrically opposite sides of the point 30.

As may be seen by reference to FIGS. 4 and 5, the flat area 24 will evert outwardly as a result of the pressure build-up so as to rupture the bottom 22 at the point 30 thereby releasing the pressure build-up. How ever, the peripheral concave area 26 of the bottom 22 does not evert. Tests have revealed that the hole at 30 is so small, as to pass only 2.3 cubic feet per minute of air at I60 p.s.i. This is insufficient to significantly move the can on a horizontal supporting surface. Also, the deformation which is shown as exaggerated is such that the can will return substantially to its undeformed position after pressure release thereby limiting product spillage by substantial closing of the hole at 30.

In accordance with this invention, the bottom 22 will rupture at a pressure no less than 170 p.s.i. and a pressure no greater than 200 p.s.i. In order to achieve pressure release, within this pressure range, various aspects of the score lines 28 (a-h) and the flat area 24 must be carefully controlled. For example, it has been found that a residual scoring of 0.002 to 0.005 results in pressure release in the range of 170 to 200 p.s.i. Actually, a scoring depth in combination with a stepped circular indentation 25 leaving a residual thickness of 0.004 inches in the region 27 of the spokes 28 and a residual thickness of 0.003 inches in the region 29 of the spokes 28 which lies beneath the indentation 25 is preferred to achieve a release pressure of 170 to I p.s.i. Preferablyi the diameter of the indentation 25 is 9/32 of an mc In order to assure the attainment of this appropriate residual thickness, a compound score line as shown in FIG. 6 is preferred; The flat area 24 is first rough scored to form a line 32 having a base 34. The base 34 is then fine scored to the appropriate residual thickness at the base 36 of a fine score line 38. It has been found that a width of 0.002 to 0.004 inches (preferably 0.0025 inches) at the base 36 and an angle 40 to 60 (preferably 50) between walls 40 of the lines 38 will result in score lines which will reliably release the pressure at the appropriate level.

Although the compound score line is preferred, a conventional score line shown in FIG. 7 may be utilized if the appropriate residual thickness can be achieved. Again, it has been found desirable to provide a 40 to 60 angle between the walls 42 with 50 preferred and a width of 0.002 to 0.004 inches with 0.0025 inches preferred at the base 44.

It is not only important to assure release before the pressure build-up reaches a level at which the double seamed bottom may unravel and evert, it is also important to avoid premature release of pressure by the valve. In this connection, it has been found that locating the score lines 28 on the exterior or outside of the bottom 22 provides reliable release before the danger level is reached while permitting the maximum residual thickness. Locating the score lines on the exterior of the bottom may result in release at a pressure of p.s.i. lower than locating the same score lines having the same residual thickness on the interior of the bottom.

As shown in FIGS. 4 and 5, a very small opening is created at point 30 when the bottom 22 ruptures at the score lines 28. This is extremely important since the score lines are located at the bottom of the aerosol can and the bottom is usually used to support the can on a horizontal surface. If the rate of flow through the opening is excessive, the can will be upset or perhaps even propelled off the horizontal surface. It has been found that the spoke-like scoring on a flattened area of the aerosol can bottom will produce an opening having a limited flow rate which is not sufficient to upset the can on the horizontal surface since the expelled fluid is able to seep between the horizontal surface and a double seam of the bottom 22 at a sufficient rate to accommodate the rate of flow through the opening.

The six spoke configuration for the valve of FIG. 8 is similar to the eight spoke scoring configuration in FIG. 3. Preferably, the flattened area 24' for the sixspoke valve and the flattened area 24 of the eight-spoke valve have 3/4 inch diameters where 135 lb. T-5 steel is utilized to achieve release at 180 p.s.i. The doming radius is 2.319 in. i 0.050 inches and the overall diameter of the domed bottom as measured from the interior of the double seam is approximately 2.553 inches for both embodiments so that the flat area in both embodiments has a diameter of less than one third the diameter of the bottom.

A method for making a safety valve comprising the compound score lines 28 in the centrally located flat area 24 will now be described with reference to FIGS. 9 (a-e). FIG. 9a shows a portion of a domed bottom 22a prior to flattening and scoring. FIG. 9b shows an anvil 50 in combination with a punch 52 for rough scoring and simultaneously flattening the central area of the bottom 22a. The bottom 22a is held between the supporting surface 54 of the anvil 50 and the scoring surface 56 on the punch 52 to form the rough scored and centrally flattened bottom 22b as shown in FIG. 90. In some instances, it maybe desirable to flatten and score the spokes in separate steps.

The rough scored and'flattened bottom 22b then assumes a position between the fine scoring punch 60 and an anvil 58. When the fine scoring surface 64 of the punch 60 closes on the bottom 22b which rests on a stepped anvil surface 62 of the anvil 58, the fine scored bottom 22c as shown in FIG. 9e is obtained.

An eight-spoke configuration for the rough scoring surface 56 is shown in FIG. 10. Flats 68 form the base 34 of the rough score lines while inclined surfaces 66 form the walls of the rough score lines. An eight-spoke fine scoring surface 64 is shown in FIG. 11. Flats 70 form the base 36 of the fine score line while inclined surfaces 72 form the walls 40 of the fine score lines as shown in FIG. 6. Finally, FIG. 12 shows a plan view of the stepped anvil 62. The central stepped area 64 of the anvil 63 permits the residual thickness of the score lines in the region 29 located beneath this stepped area to have a lesser residual thickness than the area 27 of the score lines.

In accordance with another embodiment of this invention as shown in FIGS. 13 and 14, the aerosol can of FIG. 1 is provided with a safety valve comprising a bulging or bubble-like area 124 which is centrally located in the domed bottom 22 and surrounded by an outwardly concave peripheral area 126. The several score lines 128 (a-h) in the bubble-like area 124 meet at a common point to form a spoke-like design. The score line pairs 128a and 128e, l28b and l28f, 128:: and 128g, and 128d and 128h are located on diametrically opposite sides of the point 130.

In order to render the area 124 less resistant to everting than'the surrounding area 126, the area 124 is circumscribed by a line of weakness in the form of a circular score line 132. As the pressure within the can builds up, the area 124 will evert outwardly as a result of the pressure build-up as shown in phantom in FIG. 13 so as to rupture the bottom 22 only at the point 130 thereby releasing the pressure build-up. However, the surrounding area 126 at the bottom 22 which is more resistant to everting than the area 124 will not evert.

Again, in accordance with this invention, the bottom 22 will rupture at a pressure no less than 170 p.s.i. and a pressure no greater than 200 p.s.i. In order to achieve the pressure release within this pressure range, the various aspects of the valve must be carefully controlled.

For example, the residual thickness of the score lines 128 (a-h) should be 0.002 to 0.005. Actually, it has been found that a residual thickness of 0.004 inches in the region 127 of the spokes or lines 128 and a residual thickness of 0.003 inches in the region 129 of the spokes 128 which lies beneath a 9.32 diameter circular indentation 125 is preferred to achieve a pressure release of 170 to 185 p.s.i. (see also FIG. 15c) The line of weakness 132 circumscribing the area 124 is achieved by scoring to a residual thickness of 0.010 inches. Thus the score line 132 has a greater residual thickness than the score lines 128 (a-h) although this is not indicated in FIG. 150 or l5e.

Another critical aspect of the valve shown in FIGS. 13 and 14 involves the dimensions of the bubble-like area 124. It has been found that an overall bubble diameter of 34 inch and a bubble radius 'of one inch are particularly effective. This will leave an overall bubbleheight of approximately I/ l 6 inch. The foregoing specifications apply to a bottom 22 of lb. T-5 steel (0.015 inches thick). The doming radius is again 2.319 inch 0.050 inches and the overall diameter of the domed bottom as measured from the interior of the double-seam is approximately 2.553 inches.

A method of making a safety valve comprising the bulging or bubble-like area circumscribed by a line of weakness and having score lines located inwardly of the line of weakness will now be described with reference to FIGS. 15 (a-e). FIG. 15a shows a portion of a domed bottom 122a before scoring. FIG. 15b shows a flattening and scoring punch 152 having a scoring surface 153 in combination with an anvil having a stepped supporting surface 151 for scoring the bottom 122a to form score lines 128 a-h, 130 and 132 as shown in FIG. 15c. After scoring, the bottom 1220 is placed between a doming die 156 and an anvil 154 to form the bulging having a 0.750 inch diameter is scored on the exterior of the domed can bottom 222 so as to form five substantially linear score lines 226 which extend radially outwardly from a centrally located point 228. The flat area 224 is also scored so as to form a plurality of arcuate score lines 230 located radially outwardly from the centrally located point 228 in a broken circular array.

It can be seen that each of the arcuate score lines 230 is located between an adjacent pair of linear score lines 226 at the radial outward extremities of the linear score lines. It will also be seen that the ends 232 of each arcuate score line 230 are spaced a substantial distance from the adjacent linear score lines 226 as contrasted with the embodiment shown in FIG. 14 wherein the continuous circular score line 132 is joined with radially extending score lines 128 (a-h). v

A scoring punch configuration which may be utilized to form the-valve of FIGS. 16 and 17 is shown in FIGS. 18-20. The fine scoring surface whichmay besubstituted for the surface 64in FIG. 9d comprises flats 232 to form the base 234 of the score lines while inclined surfaces 236 form the walls 238. By utilizing a stepped supporting anvil having a 9/32 inch central pedestal, the central area of the score line may be provided with a 0.003 inch residual thickness while the residual thickness of the remainder of the score line ranges from 0.005 to 0.006 inches. The scoring provides flat area 224 with. a fliplike action at pressures .in the range of 172 to 200 p.s.i. This in turn will rupture the linear score line 226 within the 9/32 inch diameter.

Although a particular embodiment of the invention has been shown and described, it will be understood that various modifications may be made in that embodiment without departing from the true spirit and scope of the invention as set forth in the appended claims. For example the bottom of the can may be entirely flat.

What is claimed is:

1. An aerosol can comprising a side wall portion, a top portion adapted to receive a valve assembly including a valve cup, and a steel bottom portion, said bottom portion having a peripheral concave area, a centrally located substantially flat area connected to and integrally formed therewith and substantially less resistant to internal deforming pressures than said peripheral concave area, and a pressure release valve formed by a plurality of intersecting score lines located on said flat area, said flat area deforming outwardly at a pressure in excess of 170 p.s.i., but below 200 psi. with resulting rupturing of said bottom portion substantially limited to a point of intersection of said score lines located substantially centrally in said flat area so as to release the contents of said can through a small opening created by said rupturing, said flat area deforming outwardly before rupturing at said point of intersection without substantial everting of said concave area.

2. The aerosol can of claim 1 wherein each of said score lines is linear.

3. The aerosol can of claim 2 wherein said score lines include pairs of lines located on diametrically opposite sides of a point on said flat area.

4. The aerosol can of claim 3 wherein said score lines are located entirely within said'flat area.

5. The aerosol can of claim 4 wherein said score lines consist of six equally angularly spaced score lines.

6. The aerosol can of claim 4 wherein said score lines consist of eight equally angularly spaced score lines.

7. The aerosol can of claim 1 wherein each said score line is a compound score line including a rough score line and a finished score line within said rough score line, said bottom portion having a residual thickness of 0.002 to 0.005 inches at the base of said fine score line.

8. The aerosol can of claim 1 wherein each of said score lines has a lesser residual thickness near the center of the flat area than at the edge of the flat area.

9. The aerosol can of claim 1 wherein the maximum. diameter of said flat area is less than one third the diameter of said bottom portion.

10. The aerosol can of claim 2 comprising a series of arcuate score lines located radially outwardly from said point of intersection in a broken circular array, each of said arcuate score lines being located between each adjacent pair of said linear score lines.

11. The aerosol can of claim 10 wherein the ends of said arcuate score lines are spaced a substantial distance from each said adjacent pair of said linear score lines.

12. The aerosol can of claim 11 wherein said arcuate score lines are substantially aligned with the ends of said linear score lines.

13. The aerosol can of claim 12 comprising five of said linear score lines and five of said arcuate score lines.

14. An aerosol can comprising a side wall portion, a top portion adapted to receive a valve assembly including a valve cup, and a steel bottom portion, said bottom portion having a peripheral concave area, a centrally located bubble-like area integrally formed with said peripheral concave area and substantially circumscribed by a score line, said buble-like area having a lesser radius of curvature than said peripheral concave area, and a pressure release valve formed by a plurality of intersecting score lines located within said bubble-like area, said bubble-like area everting at a pressure in excess of psi. with resulting rupturing of said bottom portion at the intersection of said score lines so as to release the contents of said can through an opening created by said rupturing.

15. The aerosol can of claim 14 wherein said score lines consist of eight equally angularly spaced score lines.

16. The aerosol can of claim 14 wherein said score line circumscribing said bubble-like area is substantially circular.

17. The aerosol can of claim 16 wherein the maximum diameter of said bubble-like area is less than one third the diameter of said bottom portion.

18. An aerosol can comprising a steel side wall portion, a top portion adapted to receive a valve assembly including a valve cup, and a steel domed bottom portion, said bottom portion including a steel concave peripheral area and a steel central area integral with said peripheral area and having a lesser resistance to internal deforming forces than said peripheral area, said central area having score lines intersecting in a region located substantially inwardly from said concave peripheral area, said central area deforming outwardly at a predetermined pressure while said peripheral area remains concave and substantially undeformed, said scored region rupturing at a point of intersection of said score lines after and as a result of the outward deformation of said central area without any substantial rupturing along the length of said score lines or substantial outward deformation of said peripheral area.

19. An aerosol can comprising a side wall portion, a top portion adapted to receive a valve assembly including a valve cup, and a steel bottom portion, said bottom portion having a peripheral concave area characterized by a first radius of curvature and a centrally located concave valve area characterized by a second radius of curvature different from said first radius of curvature, said concave valve area comprising a plurality of intersecting score lines everting at a pressure in excess of p.s.i. and less than 200 p.s.i. with resulting rupturing of said bottom portion at the intersection of said score lines after said everting of said concave valve area so as to release the contents of said can through an opening created by said rupturing without substantial everting of said peripheral concave area.

20. The aerosol can of claim 19 wherein said concave valve area and said score lines substantially limit the rupturing of said score lines to the intersection thereof. 

1. An aerosol can comprising a side wall portion, a top portion adapted to receive a valve assembly including a valve cup, and a steel bottom portion, said bottom portion having a peripheral concave area, a centrally located substantially flat area connected to and integrally formed therewith and substantially less resistant to internal deforming pressures than said peripheral concave area, and a pressure release valve formed by a plurality of intersecting score lines located on said flat area, said flat area deforming outwardly at a pressure in excess of 170 p.s.i., but below 200 p.s.i. with resulting rupturing of said bottom portion substantially limited to a point of intersection of said score lines located substantially centrally in said flat area so as to release the contents of said can through a small opening created by said rupturing, said flat area deforming outwardly before rupturing at said point of intersection without substantial everting of said concave area.
 2. The aerosol can of claim 1 wherein each of said score lines is linear.
 3. The aerosol can of claim 2 wherein said score lines include pairs of lines located on diametrically opposite sides of a point on said flat area.
 4. The aerosol can of claim 3 wherein said score lines are located entirely within said flat area.
 5. The aerosol can of claim 4 wherein said score lines consist of six equally angularly spaced score lines.
 6. The aerosol can of claim 4 wherein said score lines consist of eight equally angularly spaced score lines.
 7. The aerosol can of claim 1 wherein each said score line is a compound score line including a rough score line and a finished score line within said rough score line, said bottom portion having a residual thickness of 0.002 to 0.005 inches at the base of said fine score line.
 8. The aerosol can of claim 1 wherein each of said score lines has a lesser residual thickness near the center of the flat area than at the edge of the flat area.
 9. The aerosol can of claim 1 wherein the maximum diameter of said flat area is less than one third the diameter of said bottom portion.
 10. The aerosol can of claim 2 comprising a series of arcuate score lines located radially outwardly from said point of intersection in a broken circular array, each of said arcuate score lines being located between each adjacent pair of said linear score lines.
 11. The aerosol can of claim 10 wherein the ends of said arcuate score lines are spaced a substantial distance from each said adjacent pair of said linear score lines.
 12. The aerosol can of claim 11 wherein said arcuate score lines are substantially aligned with the ends of said linear score lines.
 13. The aerosol can of claim 12 comprising five of said linear score lines and five of said arcuate score lines.
 14. An aerosol can comprising a side wall portion, a top portion adapted to receive a valve assembly including a valve cup, and a steel bottom portion, said bottom portion having a peripheral concave area, a centrally located bubble-like area integrally formed with said peripheral concave area and substantially circumscribed by a score line, said buble-like area having a lesser radius of curvature than said peripheral concave area, and a pressure release valve formed by a plurality of intersecting score lines located within said bubble-like area, said bubble-like area everting at a pressure in excess of 170 p.s.i. with resulting rupturing of said bottom portion at the intersection of said score lines so as to release the contents of said can through an opening created by said rupturing.
 15. The aerosol can of claim 14 wherein said score lines consist of eight equally angularly spaced score lines.
 16. The aerosol can of claim 14 wherein said score line ciRcumscribing said bubble-like area is substantially circular.
 17. The aerosol can of claim 16 wherein the maximum diameter of said bubble-like area is less than one third the diameter of said bottom portion.
 18. An aerosol can comprising a steel side wall portion, a top portion adapted to receive a valve assembly including a valve cup, and a steel domed bottom portion, said bottom portion including a steel concave peripheral area and a steel central area integral with said peripheral area and having a lesser resistance to internal deforming forces than said peripheral area, said central area having score lines intersecting in a region located substantially inwardly from said concave peripheral area, said central area deforming outwardly at a predetermined pressure while said peripheral area remains concave and substantially undeformed, said scored region rupturing at a point of intersection of said score lines after and as a result of the outward deformation of said central area without any substantial rupturing along the length of said score lines or substantial outward deformation of said peripheral area.
 19. An aerosol can comprising a side wall portion, a top portion adapted to receive a valve assembly including a valve cup, and a steel bottom portion, said bottom portion having a peripheral concave area characterized by a first radius of curvature and a centrally located concave valve area characterized by a second radius of curvature different from said first radius of curvature, said concave valve area comprising a plurality of intersecting score lines everting at a pressure in excess of 170 p.s.i. and less than 200 p.s.i. with resulting rupturing of said bottom portion at the intersection of said score lines after said everting of said concave valve area so as to release the contents of said can through an opening created by said rupturing without substantial everting of said peripheral concave area.
 20. The aerosol can of claim 19 wherein said concave valve area and said score lines substantially limit the rupturing of said score lines to the intersection thereof. 